Our house rule--if you're not in school, you're paying 100% of your fair share. Kids in school are either too young to earn more than their share of discretionary spending/personal wants or they're focusing on college studies/expenses, i.e. studying hard to earn high grades for internships/scholarships/admission to the next level and/or earning to avoid student debt. Once the education cycle is over, it's time to start taking responsibility. We'll help when we feel it's absolutely necessary and will think nothing of it afterwards, but you better know that such a financial subsidy comes after much deliberation and with a cost--expectations, explanations, and accountability.
Parents who need to move back in with the children for non-medical reasons are likely suffering the consequences of a lifetime of bad choices, similar to a financial heart attack. I would stipulate in no uncertain terms that everyone contributes personally and financially at a level that is approximately proportional to that person's share in the expenses. In previous generations, the parents that moved in with children had paid-off property to sell and were usually beginning to slip in the maintenance of their home and possibly their health. I will always respect adult kids who take care of their ailing parents. I wouldn't feel nearly as responsible or obligated when the parent is still not retired, in good health, and earning enough that you're raising eyebrows at her monthly expenditures.
...and this doesn't even include what a spouse would say about such an in-law.